Imaging devices, such as optical scanners, are well-known in the art and produce machine-readable image data signals that are representative of a scanned object, such as a photograph or a page of printed text. In a typical scanner application, the image data signals produced by an optical scanner may be used by a personal computer to reproduce an image of the scanned object on a suitable display device, such as a CRT or a printer.
A typical optical scanner includes illumination and optical systems to illuminate the object and to focus a small area of the illuminated object, usually referred to as a "scan line," onto the surface of a photosensitive detector positioned within the scanner. Image data representative of the entire object may then be obtained by sweeping the illuminated scan line across the entire object, either by moving the object with respect to the illumination and optical assemblies or by moving the illumination and optical assemblies relative to the object. The illumination system may include a suitable white light source (e.g., a fluorescent or incandescent lamp or an array of light emitting diodes (LEDs)) to illuminate the object, whereas the optical system may include a lens assembly to focus the image of the illuminated scan line onto the surface of the detector.
The photosensitive detector array used to detect the image light focused thereon is typically a charge-coupled device (CCD), although other devices may also be used. A typical CCD may comprise a large number of individual cells or "pixels," each of which collects or builds-up an electrical charge in response to exposure to light. Since the size of the accumulated electrical charge in any given cell or pixel is related to the intensity and duration of the light exposure, a CCD may be used to detect light and dark spots on an image focused thereon. In a typical scanner application, the charge built up in each of the CCD cells or pixels is measured and then discharged at regular intervals, known as sampling intervals, which may be about 5 milliseconds or so for a typical scanner, although other sampling intervals may be used.
The term "image light" as used herein refers to the light that is focused onto the surface of the detector array by the lens assembly. Depending on the type of scanner and the type of document, the image light may be reflected from the document or object being scanned or it may be transmitted through the object or document. The image light is converted into digital signals in essentially three steps. First, each pixel in the CCD detector converts the light it receives into an electric charge. Second, the charges from the pixels are converted into analog voltages by an analog amplifier. Finally, the analog voltages are digitized by an analog-to-digital (A/D) converter. The digital signals may then be processed and/or stored as desired.
A hand-held or portable optical scanner is an optical scanner which is designed to be moved by hand across a scanned object, e.g., a page of text or a drawing or photograph. Such hand-held or portable optical scanners are well-known in the art and various components thereof are disclosed in U.S. Pat. No. 5,552,597 of McConica, and U.S. Pat. No. 5,586,212 of McConica, et al, which are hereby incorporated by reference for all that they disclose.
One problem associated with many hand-held scanners relates to the ability to maintain the appropriate positional relationship between the illumination optical system and image optical systems and the object being scanned. Generally speaking, it is desirable to maintain the optical system at right angles to the surface of the object, although other angles may be used depending on the particular design of the scanner. In any event, if the illumination optical system and, particularly, the image optical systems, are not maintained at the proper orientation, the result can be an out of focus image on the detector with a commensurate decrease in scanned image quality. In cases of extreme positional deviation, the resulting image data may be useless. If the hand-held scanner includes a position sensing system such positional deviations may compromise the effectiveness of that system as well.
To illustrate the foregoing problems, a hand-held scanner having a position sensing system may be capable of relatively high scanning speeds (e.g., 18 inches/sec). At such high scanning speeds it is difficult for the user to maintain the proper scanner orientation, particularly when changing scan directions, such as is required at the edge of the document.
The foregoing positional deviation issue does not present a serious problem in desktop and/or flat-bed scanners since the mechanical arrangement of the scanners usually precludes any significant positional deviations between the optical system and the object being scanned. However, the same is not true for many hand-held scanners. That is, since a hand-held scanner is moved by hand across the object being scanned, there is a significant likelihood that the user will induce significant positional deviations by rocking and/or tilting the scanner while sweeping it over the object. While skillful users may be able to sweep the hand-held scanner over the object without significant positional deviations, the scanning process can be considerably more difficult and frustrating for less-skilled and/or novice hand-held scanner users.
Therefore, a need exists for a hand-held scanner that will allow a user to more easily maintain the proper positional relationship between the scanner optics and the object being scanned in order to more consistently produce high-quality scanned image data.
One solution to maintain a constant optical path length is a Floating Image Head Design, such as that disclosed in U.S. Pat. No. 5,923,444 of Bohn, which is incorporated herein for all that it teaches. The disadvantage of the Floating Image Head Design is that all of the optics are contained in the rotating image head that requires the use of a SELFOC type lens, which has a poor field depth and poor color imaging and is quite expensive. It would be advantageous to have a hand-held scanner that maintains a constant optical path length without the required use of an expensive SELFOC type lens.